Testing of structural materials for low temperatures



H. A. BARTH July 14, 1964 TESTING OF STRUCTURAL MATERIALS FOR LOW TEMPERATURES 3 Sheets-Sheet 1 Original Filed Nov. 17, 1959 322 2 ozazmm m0 mmmwmn E61 ad; 8

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H. A. BARTH July 14, 1964 TESTING OF STRUCTURAL MATERIALS FOR LOW TEMPERATURES Original Filed Nov. 17, 1959 3 Sheets-Sheet 3 025 5 mwmzzwnoh 53 nuxukoz 53. s 35 a 3 2953mm QwQmOm Phi mama

United States Patent 3,140,602 TESTING OF STRUCTURAL MATERIALS FOR LOW TEMPERATURES Heinz Alfred Barth, Pullach im Isartal, Germany, assignor to Gesellschaft fiir Lindes Eismaschinen Aktiengesellschaft, Hollriegelskreuth, near Munich, Germany, a company of Germany Original application Nov. 17, 1959, Ser. No. 853,599. Divided and this application Mar. 17, 1961, Ser. No. 96,504 Claims priority, application Germany Nov. 21, 1958 1 Claim. (Cl. 73100) At low temperatures most materials assume more or less abnormal properties, as known. In particular, many materials at below 0 C., particularly, below -50 C., to a great extent lose the properties which characterize them as structural materials. For this reason copper or V2A-steel, as well as a few special alloys which have been specially developed for certain uses and certain temperature ranges, are used in refrigeration engineering for the construction of apparatus, particularly tanks, gasand air-decomposition apparatus, etc., which are operated preferably at temperature below -l00 C., for example, at the temperatures of liquid air or of liquid hydrogen. V2A-steel is expensive and hard to machine, while copper has only low strength values and must, therefore, be processed and used in great wall thicknesses.

The object underlying the invention essentially consists in permitting the use of other materials in refrigeration engineering which are superior to copper in strength and are better, and easier to machine, than V2A steel.

The investigations underlying this invention, particularly measurements of the material properties important for refrigeration on various alloys and metals that have already been used for normal temperatures, surprisingly have shown that brass 58 (an alloy of copper and zinc containing 58% copper) is suitable as a material for use at low temperatures, particularly below 50 C., especially at the temperatures of liquid air and liquid hydrogen, and even down to the temperature of liquid helium, particularly as rolled or molded brass, provided it has a higher degree of purity than the ordinary brass 58 and is specially selected with regard to fineness and to uniformity of the grain.

According to the invention, the brass 58 used as a structural material in apparatus exposed to temperatures below 0 C., particularly below 50 C., has the following composition:

Copper, 58.0, 60.0% by weight.

Iron, less than 2.0, preferably less than 1.2% Manganese, less than 2.0, preferably less than 1.2%. Aluminum, less than 0.2, preferably less than 0.02%.

Lead, less than 0.2, preferably less than 0.02%. Balance, zinc.

The zinc used must not contain more than 0.22% by weight antimony and not more than 0.02% by weight bismuth. Up to about 5% of the zinc can be replaced by nickel, and up to 2% of the copper by nickel, but altogether there must not be more than 5% nickel present in the alloy.

By observing these conditions it is possible to impart to the brass definite strength values particularly as regards the yield point. New is the requirement to achieve a minimum notch impact strength in brass 58 and simultaneously to ensure to be able to make, of brass 58, structural parts with a greater notch sensitivity at lower temperature. But this should not be done at the expense of the elastic behavior at low temperatures. Besides an optimum of stress-corrosion insensitivity in undrawn and molded semi-products, respectively, of brass 58 should be achieved.

FIG. 1 shows the characteristic of such a brass 58 according to the invention which meets the requirements of maximum ductility at lowest temperatures.

The deformability of the material used at low temperatures determines to a considerable extent the usefulness of a whole part. It is, therefore, not advisable to make the evaluation of the suitability dependent only on a single material characteristics, for example, on ductility alone.

Since a chemical and metallurgical test of the material to be used and provided respectively in brass 58 is difiicult to effect, because of the high purity required according to the invention, a new testing method has been developed according to a special development of the inventive concept, which permits a relatively simple quality test of the material in the molded or drawn state for the intended use.

The test is elfected in this way, that a threaded shaft is cut from a sample of a brass 58 material selected according to the invention, which has, for all samples of this type, a uniform design with regard to depth of cut, pitch, etc., and which in particular always has the same length, and this threaded shaft is subsequently subjected to a bending test which is known in itself. The first crack in the base of the threaded shaft is observed during the bending. The appearance of the first crack determines the degree of possible deformation and the bending angle. In this way it is possible to differentiate a number of brass alloys 58 which yield practically the same results in the customary +20 C. normal test.

FIG. 2 shows a comparison of the bending angle after the above-described thread-bending test and the notch impact strength; the graph shows the separation of the types (quality differentiation) of some brass 58 samples tested according to this method. Type B is the prototype represented in FIG. 1 as it was developed corresponding to the above-described requirements.

FIG. 3 shows one of the above-described thread-bending tests and the respective notch impact test at the temperatures of liquid nitrogen. In order to evaluate the question as to what extent brass 58 according to the invention can be used in low temperature apparatus in the molded or forged state, FIG. 4 shows a few results obtained by a comparison of the values of the edge and core zones in molded and forged brass 58 according to the invention of the same types of alloys.

For the measurement of castings substantially the same viewpoints as set forth above apply, if one starts from the type GMs 57.

This application is a division of applicants copending application Serial No. 853,599, filed November 17, 1959.

I claim:

Process for testing the ductility of a test sample of metal, which comprises making a shaft-shaped test sample of the metal, forming a thread on it of predetermined standard length and depth of groove and pitch, clamping said sample at its ends to support said sample, applying force to the said sample by means of said clamps so as to bend the sample in an arc, observing the appearance of the first crack in the threaded area, and measuring the precise bending angle defined by the flexed sample at that moment.

References Cited in the file of this patent UNITED STATES PATENTS 2,283,730 Gardner May 19, 1942 

